1. Field of the Invention
The present invention pertains to a method of forming a titanium-comprising or a tantalum-comprising barrier/wetting layer structure. This structure is useful when a series of semiconductor substrates is to be fabricated in a process chamber, as it enables a consistently high degree of &lt;111&gt; crystal orientation in an aluminum interconnect layer or a copper interconnect layer deposited over such a barrier/wetting layer structure, throughout the processing of the series of substrates.
2. Brief Description of the Background Art
Titanium nitride layers have been used in semiconductor device structures as barrier layers for preventing the interdiffusion of adjacent layers of materials such as aluminum and silicon, for example. However, the crystal orientation of aluminum deposited over the surface of the titanium nitride barrier layer is typically polycrystalline, and polycrystalline aluminum has poor electromigration resistance.
In the formation of integrated circuit interconnect structures, such as a Ti/TiN/TiN.sub.x /Al stack, electromigration of aluminum atoms within the aluminum layer becomes a problem if the aluminum layer is not formed with a high degree of &lt;111&gt; crystal orientation. Electromigration of the aluminum atoms can result in open circuits within the integrated circuit structure, and therefore, such electromigration must be inhibited or eliminated. Electromigration of aluminum atoms can occur within filled vias as well, impairing the conductivity of the contacts.
U.S. Pat. No. 4,944,961 to Lu et al., issued Jul. 31, 1990, describes a process for partially ionized beam deposition of metals or metal alloys on substrates, such as semiconductor wafers. Metal vaporized from a crucible is partially ionized at the crucible exit, and the ionized vapor is drawn to the substrate by an imposed bias. Control of substrate temperature is said to allow non-conformal coverage of stepped surfaces such as trenches or vias. When higher temperatures are used, stepped surfaces are planarized. The examples given are for aluminum deposition, where the non-conformal deposition is carried out with substrate temperatures ranging between about 150.degree. C. and about 200.degree. C., and the planarized deposition is carried out with substrate temperatures ranging between about 250.degree. C. and about 350.degree. C.
S. M. Rossnagel and J. Hopwood describe a technique of combining conventional magnetron sputtering with a high density, inductively coupled RF plasma in the region between the sputtering cathode and the substrate in their 1993 article titled "Metal ion deposition from ionized magnetron sputtering discharge", published in the J. Vac. Sci. Technol. B. Vol. 12, No. 1, Jan/Feb 1994. One of the examples given is for titanium nitride film deposition using reactive sputtering, where a titanium cathode is used in combination with a plasma formed from a combination of argon and nitrogen gases.
U.S. Pat. No. 5,262,361 to Cho et al., issued Nov. 16, 1993 describes a method for forming single crystal aluminum films on the surface of a substrate such as silicon (111). The object is to increase the amount of the aluminum (111) crystal orientation, to improve the electromigration resistance of the aluminum. Electrically neutral aluminum is deposited by a vacuum evaporation technique upon a silicon wafer surface at a temperature ranging between about 300.degree. C. and about 400.degree. C.
U.S. Pat. No. 5,543,357 to Yamada et al., issued Aug. 6, 1996, describes a process for manufacturing a semiconductor device wherein a titanium film is used as an under film for an aluminum alloy film to prevent the device characteristics of the aluminum alloy film from deteriorating. The thickness of the titanium film is set to 10% or less of the thickness of the aluminum alloy film and at most 25 nm. In the case of the aluminum alloy film containing no silicon, the titanium film is set to 5% of less of the thickness of the aluminum alloy film. The aluminum film is formed at a substrate temperature of 200.degree. C. or less by a sputtering process, and when the aluminum film or an aluminum alloy film is to fill a via hole, the substrate is heated to fluidize the aluminum. The pressure during the aluminum film formation and during the fluidization is lower than 10.sup.-7 Torr. A titanium nitride barrier layer may be applied on an interlayered insulating film (or over a titanium layer which has been applied to the insulating film), followed by formation of a titanium film over the titanium nitride film, and finally by formation of the aluminum film over the titanium film. After formation of the titanium nitride barrier layer, the barrier layer is heated to a temperature of about 600.degree. C. to 700.degree. C. in a nitrogen atmosphere using a halogen lamp so that any titanium which is not nitrided will become nitrided. The titanium nitride barrier layer is said to be a poor barrier layer if un-nitrided titanium is present within the layer.
U.S. Pat. No. 5,571,752 to Chen et al., issued Nov. 5, 1996, discloses a method for patterning a submicron semiconductor layer of an integrated circuit. In one embodiment, titanium or titanium nitride having a thickness of between approximately 300 and 2000 .ANG. is formed by sputter deposition to reach the bottom of a contact opening. The barrier layer may be annealed to form a silicide in the bottom of the opening. A conformal conductive layer of a refractory metal or refractory metal silicide is formed over the titanium or titanium nitride using chemical vapor deposition (CVD). Finally, a second conductive layers typically aluminum is applied over the surface of the conformal conductive layer. The aluminum is sputtered on, preferably at a temperature ranging between approximately 100.degree. C. and 400.degree. C. This method is said to make possible the filling of contact openings having smaller device geometry design requirements by avoiding the formation of fairly large grain sizes in the aluminum film.
U.S. patent application, Ser. No. 08/753,251 of Ngan et al., filed Nov. 21, 1996, describes a method for producing a titanium nitride-comprising barrier layer on the surface of a contact via. For certain contact geometries, when the reactor pressure is reduced during formation of the titanium nitride-comprising barrier layer, the thickness of the barrier layer on the sidewalls of the via increases. This enables an aluminum fill to travel along the sidewalls of the via more easily, resulting in a better fill of the via. In particular, the titanium nitride comprising barrier layer needs to be of a minimum thickness and to have a minimum titanium content so that the barrier layer can react slightly with the Aluminum, to draw the aluminum along the sidewalls of the via.
U.S. patent application, Ser. No. 08/511,825 of Xu et al., filed Aug. 7, 1995, assigned to the Assignee of the present invention, and hereby incorporated by reference in its entirety, describes a method of forming a titanium nitride-comprising barrier layer which acts as a carrier layer. The carrier layer enables the filling of apertures such as vias, holes or trenches of high aspect ratio and the planarization of a conductive film deposited over the carrier layer at reduced temperatures compared to prior art methods.
A "traditionally sputtered" titanium nitride-comprising film or layer is deposited on a substrate by contacting a titanium target with a plasma created from an inert gas such as argon in combination with nitrogen gas. A portion of the titanium sputtered from the target reacts with nitrogen gas which has been activated by the plasma to produce titanium nitride, and the gas phase mixture contacts the substrate to form a layer on the substrate. Although such a traditionally sputtered titanium nitridecomprising layer can act as a wetting layer for hot aluminum fill of contact vias, good fill of the via generally is not achieved at substrate surface temperature of less than about 500.degree. C. To provide for aluminum fill at a lower temperature, Xu et al. (as described in U.S. patent application, Ser. No. 08/511,825), developed a technique for creating a titanium nitride-comprising barrier layer which can act as a smooth carrier layer, enabling aluminum to flow over the barrier layer surface at lower temperatures (at temperatures as low as about 350.degree. C., for example). A typical barrier layer described by Xu et al., is a combination of three layers including a first layer of titanium (Ti) deposited over the surface of the via; a second layer of titanium nitride (TiN) is deposited over the surface of the first titanium layer; finally a layer of TiN.sub.x is deposited over the TiN second layer. The three layers are deposited using Ion Metal Plasma (IMP) techniques which are described subsequently herein. Typically the first layer of titanium is approximately 100 .ANG. to 200 .ANG. thick; the second layer of TiN is about 800 .ANG. thick, and the third layer of TiN.sub.x is about 60 .ANG. thick. Although a good fill of contact vias having 0.25.mu. diameter through holes having an aspect ratio of about 5 was achieved, the crystal orientation of the aluminum was low in {111} crystal orientation content, resulting in poor electromigration (EM) performance for the aluminum interconnect. It was desired to increase the aluminum {111} crystal orientation content for purposes of improving the EM performance.
U.S. patent application, Ser. No. 08/825,216 of Ngan et al., filed Mar. 27, 1997, discloses various process techniques which can be used to control the crystal orientation of a titanium nitride barrier layer as it is deposited.
U.S. patent application, Ser. No. 08/824,911 of Ngan et al., filed Mar. 27, 1997 discloses improved Ti/TiN/TiN.sub.x barrier/wetting layer structures which enable the aluminum filling of high aspect vias while providing an aluminum fill exhibiting a high degree of aluminum {111} crystal orientation. In particular, an improved Ti/TiN/TiN.sub.x barrier layer deposited using IMP techniques can be obtained by increasing the thickness of the first layer of Ti to range from greater than about 100 .ANG. to about 500 .ANG. (the feature geometry controls the upper thickness limit); by decreasing the thickness of the TiN second layer to range from greater than about 100 .ANG. to less than about 800 .ANG. (preferably less than about 600 .ANG.); and, by controlling the application of the TiN.sub.x third layer to provide a Ti content ranging from about 50 atomic percent titanium (stoichiometric) to about 100 atomic percent titanium. Preferably the TiN.sub.x third layer is formed at the end of the deposition of the TiN second layer and exhibits a Ti content gradient which begins at a stoichiometric, 50 atomic percent, Ti content and ends at a Ti content of about 100 atomic percent. The thickness of the TiN.sub.x third layer preferably ranges from about 15 .ANG. to about 500 .ANG.. The improved Ti/TiN/TiN.sub.x barrier layer enables the deposit of an aluminum interconnect an aluminum via fill where the aluminum exhibits a high {111} crystallographic content. U.S. patent application, Ser. No. 08/824,911 is hereby incorporated herein by reference, in its entirety.
Subsequent to the filing of U.S. patent application Ser. No. 08/824,911, we discovered that in a production simulation, with a cassette containing a large quantity of semiconductor wafers processed in series in a given process chamber, there were unknown factors present at the beginning of processing which affected the &lt;111&gt; crystal orientation of the aluminum layer. Although the method provided in U.S. patent application Ser. No. 08/824,911 enables the deposit of a high &lt;111&gt; crystallographic aluminum content, to ensure a consistently high aluminum &lt;111&gt; content throughout the processing of a large number of semiconductor substrates, it is necessary to either eliminate the unknown factors affecting the crystalline structure or to find a way to compensate for them.